1. Field of the Invention
This invention generally relates to a ship, in particular, to a deep-draft vessel configured for travel on the oceans and great lakes. Such a ship could be outfitted with a stern tube having two contra-rotating propellers. In such a case, the propellers will be driven by concentrically rotating propeller shafts, with a rearmost propeller connected to an inner shaft, and a forward propeller connected to a hollow outer shaft in which the inner shaft is disposed. The inner shaft and the outer shaft preferably rotate in opposite directions to one another, and the arrangement will generally include at least one bearing for the inner shaft, which bearing can preferably be disposed between the inner shaft and the outer shaft.
In general it is also necessary to provide lubricant to the bearing surfaces to reduce the friction at the surfaces between the bearing and the rotating shaft adjacent the bearing.
2. Background Information
When an inner shaft is mounted in a contra-rotating outer shaft, problems can occur unless a sufficient quantity of lubricant can be supplied to the bearing area.
A stern tube bearing system of the type described above is disclosed in European Patent No. A2-0 221 536. In the arrangement disclosed by this patent, the inner shaft is hollow, and in the vicinity of the bearing, has a number of radial holes to supply oil to the bearing surfaces. As such, oil under high pressure is transported via the cavity of the inner shaft to the oil ducts, and thence into the bearing gap between the bearing and the shaft surface adjacent the bearing. One disadvantage of such an arrangement, however, is that, due to the rotation of the inner shaft, oil is also forced into the upper portion of the bearing gap through the radial oil ducts which are distributed around the circumference of the inner shaft. In and of itself, that is not critical with regard to the lubrication, but it has an adverse effect on the oil supply in the critical lower portion of the bearing, or the portion which substantially bears the weight of the shaft.